The beauty of things like M4s and Hicapas is the customizability. Yes, everyone seems to have one, but they can be done up uniquely very easily. Whereas something like a Desert Eagle or 5-7 (or SCAR, P90, etc) are all the same, always. They cannot be customized in the same way.

Yeah, you could line up 10 guys (or 20) with their ARs, and it could very well be that no two of the ten would look alike, such is the adaptability and upgrade/mission-specific options of the AR platform.